MINERALOGY OF THE NIEDERSCHLEMA-ALBERODA U-Se-POLYMETALLIC DEPOSIT, ERZGEBIRGE, GERMANY. III. FIRST INDICATION OF COMPLETE MISCIBILITY BETWEEN TENNANTITE AND GIRAUDITE

摘要

Giraudite and hakite, and their more widespread S-dominant analogues, tennantite and tetrahedrite, are locally abundant, but overall rare species in the Niederschlema-Alberoda uranium deposit in the Erzgebirge of Germany. The continuous mercurian giraudite-hakite series, giraudite-tennantite solid solutions, and Se-rich zincian and ferroan tennantite and zincian tetrahedrite are part of the Jurassic selenide mineralization. Extensive, polished-section-scale compositional variation is observed for the giraudite-tennantite series, expressed by the following structural formula (calculated on the basis of 29 atoms per formula unit): (CU_(9.86-10.00)Ag_(0.00-0.14))(SIGMA(10) (Cu_(0.18-1.88)~(2+)Fe_(0.09-1.77)Zn_(0.00-1.26)Hg_(0.00-0.10)Cd_(0.00-0.06)(SIGMA(1.95-2.16) (As_(2.23-4.05)Sb_(0.00-1.68)Te_(0.00-0.14)Bi_(0.00-0.01)(SIGMA(3.85-4.07)(Se_(0.00-12.64)S_(0.32-12.97))(SIGMA(12.90-13.09)(n = 58). The solid solutions span the range gir_0tn_(100) to gir_(98)tn_2 with only two minor gaps, suggesting extensive, likely complete miscibility between giraudite and tennantite in nature. Giraudite may contain Hg, Cu, Zn, and Fe as the predominant divalent cation. Cretaceous tennantite, deposited together with Bi-Co-Ni-Ag minerals, is Se-deficient, contains minor amounts of Co and Ni, is enriched in Ag, and may contain appreciable amounts of Bi. Bismuthoan zincian tennantite, the replacement product of wittichenite, contains up to 12.6 wt percent Bi (equivalent to 0.97 apfu). The origin of the various tennantite-tetrahedrite mineral associations is discussed in the light of the temporal sequence and the activities of selenium and sulfur that prevailed during their formation. The Se-rich and Se-bearing sulfosalts of Jurassic age are suggested to be late-stage species, deposited from low-temperature hydrothermal fluids having an activity of selenium several orders of magnitude lower than that approached during the crystallization of umangite and klockmannite, which mark the peak in/(Se_2) reached at Niederschlema-Alberoda.